Mass Spectrometry

Courtesy www.lab-initio.com

Purpose of Mass Spectrometry

Produces spectra of masses from the molecules in a sample of material, and fragments of the molecules.

Used to determine the elemental composition of a sample the masses of particles and of molecules potential chemical structures of molecules by

analyzing the fragments the identity of unknown compounds by

determining mass and matching to known spectra

the isotopic composition of elements in a molecule

StagesThe ionizer converts some of the sample into ions.Mass analyzers separate the ions according to their mass-to-charge ratio. 

The detector records either the charge induced or the current produced when an ion passes by or hits a surface

Mass Spectrometry Methods

Ionization method

Typical Analytes

Sample Introduction

Mass Range

Electron  Impact (EI)Relatively  

small  volatile

GC or  liquid/solid  

probe

to  1,000  Daltons

Chemical Ionization (CI)

Relatively  small  volatile

GC or  liquid/solid  

probe

to  1,000  Daltons

Electrospray (ESI)Peptides  Proteins  

nonvolatile

Liquid  Chromatography  

or syringe

to  200,000  Daltons

Fast Atom Bombardment (FAB)

Carbohydrates  Organometallics  

Peptides  nonvolatile

Sample mixed  in viscous  

matrix

to  6,000  Daltons

Matrix Assisted Laser Desorption  (MALDI)

Peptides  Proteins  

Nucleotides

Sample mixed  in solid  matrix

to  500,000  Daltons

http://www.chem.arizona.edu/massspec/intro_html/intro.html

Mass Spectrum of CO2

Molecular ion peak[CO2]+ = 44

[CO]+ = 28 [O]+ = 16 [C]+ = 12

Fragment Peaks

Mass Spectrum of Bromine, Br2

Bromine has two isotopes: 50.69% 79Br and 49.31% 81Br

79Br+ 81Br+

[79Br81Br]+

[79Br79Br]+

[81Br81Br]+

Molecular Ion Peaks

Fragments

Practice: Methyl Bromide, CH3Br

Answers: Methyl Bromide, CH3Br

[CH381Br]+

[CH281Br]+

[C81Br]+ and [CH279Br]+

[CH81Br]+ and [CH379Br]+

[CH79Br]+

[CH3]+

[C79Br]+[81Br]+

[79Br]+

Practice: Methylene Chloride (CH2Cl2)Chlorine is 75.77% 35Cl and 24.23% 37Cl

Practice: Vinyl Chloride (CH2CHCl)Chlorine is 75.77% 35Cl and 24.23% 37Cl

Spectra of Larger MoleculesSpectra of large molecules have many fragments, and the interpretation of their spectra is beyond the scope of this course.

Codeine, C18H21NO3

Toluene ( C7H8) Fragmentation

**Note that the molecular ion peak is NOT the dominant peak in the spectrum

Mass Spectrometry in Forensics

Mass spectrometry is used to confirm the identify of unknowns, such as illegal drugs

Unknowns are often not pure, and must be separated from a mixture

Gas chromatography is used to separate the components of the mixture

Mass spectrometry “fingerprints” the components, so that they can be matched to existing known spectra

GasChromatographMixture

MassSpectrometer